The present invention relates to an insulated gate bipolar transistor (hereinafter abbreviated as IGBT) that is used as a power switching device.
In recent years, the IGBT has come to be commonly used as a power switching device. The IGBT has a structure in which a reverse conduction-type layer is added to the drain region of the longitudinal MOSFET at its drain electrode side. More specifically, FIG. 2 shows a unit cell of the IGBT. A high-resistivity n.sup.- layer 3 is formed on a p.sup.+ substrate 1 via a low-resistivity n.sup.+ layer 2. Further, a p layer 4 is selectively formed in a surface layer of the n.sup.- layer 3, and an n.sup.+ layer 5 is selectively formed in a surface layer of the p layer 4. A surface portion of the p layer 4 that is located between the n.sup.- layer 3 and the n.sup.+ layer 5 serves as a channel region, and a gate electrode 7, that is connected to a gate terminal G, is formed on the channel region via a gate insulating film 6. A source electrode 9, that is insulated from the gate electrode 7 by an insulating film 8 and connected to a source terminal S, is in contact with the p layer 4 and the n.sup.- layer 5. A drain electrode 10, that is connected to a drain terminal D, is in contact with the p.sup.+ substrate 1.
When a positive voltage is applied to the gate terminal G and the drain terminal D while the source terminal S is grounded, a built-in MOSFET is turned on that consists of the n.sup.+ layer 2, n.sup.- layer 3, p layer 4, n.sup.+ layer 5, gate electrode 7 and source electrode 9, so that electrons flow into the n.sup.- layer 3 via the channel region. On the other hand, holes are injected into the n.sup.- layer 3 from the p.sup.+ substrate 1 via the n.sup.+ layer 2 by a quantity corresponding to that of the electrons flowing into the n.sup.- layer 3. As a result, a conductivity modulation occurs in the n.sup.- layer 3 to reduce the resistance of this layer. Therefore, the IGBT is turned on with a low on-resistance.
While having a small on-voltage, the above-described conventional IGBT is associated with a problem that due to a low electron-hole recombination rate in the n.sup.- layer 3 the switching time is .long and therefore the switching loss is large. To solve this problem, i.e., to increase the electron-hole recombination rate, the life time may be reduced by introducing life time killers by illuminating the silicon base with an electron beam or by gold diffusion. However, these measures unavoidably increase the on-voltage of the IGBT. That are, the on-voltage and the switching time is in a tradeoff relationship, and it is very difficult to improve both characteristics at the same time.